Apparatus for driving liquid crystal display panel for small size image

Abstract: In an apparatus for driving a liquid crystal display panel (1) having N scan lines (SCi) (N=2, 3,...), shift registers (21-0, 21-1, ...) are provided to drive the scan lines, and switching circuits (22-0, 22-1, ...) are interposed among the shift registers. One of the switching circuits is selected to write a start pulse signal (ST) thereinto. Thus, an image having a smaller number of scan lines than N can be displayed at a center portion of the liquid crystal display panel.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a liquid crystal display (LCD) system, and more particularly, to an apparatus for driving a multi-synchronization type LCD panel for a small size image. Description of the Related Art There has been known a multi-synchronization type deflecting apparatus for a cathode-ray tube (CRT) panel which can properly display images having different numbers of scan lines at a center portion of the panel. On the other hand, since LCD panels are thinner in size and lower in power consumption with a lower power supply voltage as compared with CRT panels, the LCD panels have recently been applied to personal computers, word processors, color telereceivers, and the like. However, the multi-synchronization type deflecting system of the CRT panels cannot be applied to the multi-synchronization type driving system of the LCD panels, due to the difference in driving (deflecting) methods therebetween. In a prior art apparatus for driving an LCD panel having N scan lines (N=2, 3,...), N serially-connected shift registers are provided to drive the scan lines. That is, a start pulse signal, which is in synchronization with a horizontal synchronization signal, is written into the first stage of the shift registers, and the start pulse signal is shifted through the shift registers. As a result, an image having a smaller number of scan lines than N is ill-balanced at an upper portion of the LCD panel. This will be explained later in detail. EP-A-0,456,165 discloses a color LCD display control system. The display control system disclosed therein comprises a driver circuit formed by shift register which comprises serially connected D flip-flops. At two locations of the shift register there is disposed a selector circuit at the input of the respective flip-flop. Depending on the desired resolution, by the two selectors a sub-range of the driver can be selected for employing a lower display resolution. The disclosed driver is, however, not suitable for centering an image having an arbitrary small number of scan lines SUMMARY OF THE INVENTION It is an object of the present invention to provide a multi-synchronization type driving apparatus for an LCD panel which can display an image having a small number of scan lines at a center portion thereof. The present invention is defined in claim 1. Claims 2 to 8 define particular embodiments thereof. In an apparatus for driving an LCD panel having N scan lines (N=2, 3, ···), D-flip-flops forming shift registers are provided to drive the scan lines, and switching circuits are interposed among the shift registers. One of the switching circuits is selected to write a start pulse signal thereinto. Thus, an image having a smaller number of scan lines than N can be displayed at a center portion of the LCD panel. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the description as set forth below, in comparison with the prior art, with reference to the accompanying drawings, wherein: Fig. 1 is a block circuit diagram illustrating a prior art apparatus for driving an LCD panel; Fig. 2 is a detailed block circuit diagram of the scan line driving circuit of Fig. 1; Figs. 3A through 3E are timing diagrams showing the operation of the circuit of Fig. 2; Figs. 4A, 4B and 4C are timing diagrams of the image signals displayed on the LCD panel of Fig. 1; Fig. 4D is a diagram showing images displayed on the LCD panel of Fig. 1; Fig. 5 is a block circuit diagram illustrating an embodiment of the apparatus for driving an LCD panel according to the present invention; Fig. 6 is a detailed block circuit diagram of the image size determining circuit of Fig. 5; Fig. 7 is a diagram showing the content of the look-up table of Fig. 6; Fig. 8 is a detailed block circuit diagram of the scan line driving circuit of Fig. 5; Fig. 9 is a detailed circuit diagram of the switching circuit of Fig. 8; Figs. 10A through 10I are timing diagrams showing the operation of the circuit of Fig. 5; Figs. 11A, 11B and 11C are timing diagrams of the image signals displayed on the LCD panel of Fig. 5; Fig. 11D is a diagram showing images displayed on the LCD panel of Fig. 5; Fig. 12 is a block circuit diagram of one modification of the circuit of Fig. 8; Fig. 13 is a block circuit diagram of one modification of the circuit of Fig. 6; and Fig. 14 is a diagram showing the content of the look-up table of Fig. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before the description of the preferred embodiments, a prior art apparatus for driving an LCD panel will be explained with reference to Figs. 1, 2, 3A through 3E, and 4A through 4D. In Fig. 1, which illustrates a prior art apparatus for driving an LCD panel, reference numeral 1 designates an LCD panel having M × N dots where M=1280 and N=1024. That is, the LCD panel 1 has 1024 scan lines SL i (i=0, 1,..., 1023) driven by a scan line driving circuit 2, signal lines SG j (j=0, 1,..., 1279) driven by signal line driving circuits 3-1 and 3-2, and pixels each connected to one of the scan lines and one of the signal lines. Also, each of the pixels is formed by a thin film transistor (TFT) Q ij and a liquid crystal cell C ij . A signal processing circuit 4 receives color signals R, G and B, to thereby convert them by using a timing signal from a timing generating circuit 5. The output signal of the signal processing circuit 4 is supplied to the signal line driving circuits 3-1 and 3-2. The timing generating circuit 5, which includes phase-locked loop (PLL) circuits, receives a horizontal synchronization signal HSYNC and a vertical synchronization signal VSYNC, to thereby generate various timing signals for controlling the scan line driving circuits 2 and the signal line driving circuits 3-1 and 3-2 in addition to the signal processing circuit 4. For example, the timing generating circuit 5 generates a start pulse signal ST for showing the first scan line of a displayed image in synchronization with the horizontal synchronization signal HSYNC, and a shift clock signal SCK for shifting the scan line of the displayed image in synchronization with the vertical synchronization signal VSYNC. In Fig. 2, which is a detailed block circuit diagram of the scan line driving circuit 2 of Fig. 1, D flip-flops 21-0, 21-1, ···, 21-1023 are serially-connected to form a shift register for driving the scan lines SL 0 , SL 1 , SL 1023 , respectively. In Fig. 2, the start pulse signal ST as shown in Fig. 3A is supplied to the first stage of the flip-flops, i.e., the flip-flop 21-0, and the start pulse signal ST is shifted through the flip-flops 21-0, 21-1, ···, 21-1023 by the shift clock signal SCK as shown in Fig. 3B. As a result, the scan lines SL 0 , SL 1 , ··· , SL 1023 are sequentially driven by the output signals D 0 , D 1 , ···, D 1023 of the shift registers 21-0, 21-1,..., 21-1023. Therefore, even if an image having 1152 × 900 dots as shown in Fig. 4B, that is smaller than an image having 1280 × 1024 dots as shown in Fig. 4A, is displayed in the LCD panel 1 having 1280 × 1024 dots, the timing of the start pulse signal ST is definite as shown in Fig. 4C. As a result, as shown in Fig. 4D, a 1152 × 900 dot image is ill-balanced at an upper portion of the LCD panel 1. In Fig. 5, which illustrates an embodiment of the present invention, an image size determining circuit 6 is added to the elements of Fig. 1, and the scan line driving circuit 2 of Fig. 1 is modified into a scan line driving circuit 2'. The image size determining circuit 6 calculates ΔN by ΔN=(1024-N')/2 where N' is a number of scan lines of an image to be displayed on the LCD panel 1. In this case, the equation can be replaced by ΔN=(1024-f H /f V )/2 where f H is a frequency of the horizontal synchronization signal HSYNC and f V is a frequency of the vertical synchronization signal VSYNC. Therefore, the image size determining circuit 6 is formed by a circuit as illustrated in Fig. 6. In Fig. 6, reference numeral 61 designates a frequency-to-voltage converter for receiving the horizontal synchronization signal HSYNC to generate a voltage V N in response to the frequency of the horizontal synchronization signal HSYNC. Also, reference numeral 62 designates a frequency-to-voltage converter for receiving the vertical synchronization signal VSYNC to generate a voltage V V in response to the frequency of the vertical synchronization signal VSYNC. The voltages V H and V V are converted by analog-to-digital converters 63 and 64 into digital values f H and f V , respectively. Then, the digital values f H and f V are supplied to a look-up table 65, which in turn generates a 10-bit address signal ADD. Note that the look-up table 65 is formed by a random access memory (RAM) or a read-only memory (ROM) in which the values ΔN defined by the equation (2) are stored in advance. For example, the content of the look-up table 65 is shown in Fig. 7. In this case, note that the value of the address signal ADD is from "0000000000"(=0) to "0111111111"(=511). The details of the scan line driving circuit 2' of Fig. 5 are illustrated in Fig. 8. In Fig. 8, flip-flops 22-0, 22-1,..., 22-1023 and a decoder 23 are added to the elements of Fig. 2. The decoder 23 has 1024 output lines, each connected to one of the switching circuits 22-0, 22-1, ..., 22-1024. The switching circuits 22-0, 22-1,..., 22-1023 are interposed at the inputs of the flip-flops 21-0, 21-1, ···, 21-1023, respectively, and are selected by the decoder 23. That is, the decoder 23 receives the 10-bit address signal ADD to select one of the switching circuits 22-0, 22-1, ···, 21-1023, and as a result, only the selected switching circuit selects its B terminal and the other non-selected switching circuits select their A terminals. Each of the switching circuits 22-i (i=0, 1, ···, 1023) can be formed by two AND circuits 221 and 222, an inverter 223, and an OR circuit 224 as illustrated in Fig. 9. For example, if an image having 1152 x 900 dots is displayed on the LCD panel 1, the image size determining circuit 6 generates the address signal ADD whose value is (1024-900)/2 =62 (="0001111101") Therefore, the decoder 23 selects the switching circuit 22-62. As a result, only the switching circuit 22-62 selects its B terminal, and the other switching circuits select their A terminals. Therefore, the start pulse signal ST as shown in Fig. 10A is supplied directly to the flip-flop 21-62, and the start pulse signal ST is shifted by the shift clock signal SCK as shown in Fig. 10B through the shift registers 21-62 through 21-1023 as shown in Figs. 10F, 10G, 10H and 10I. In this case, the start pulse signal ST is never written into the shift registers 21-0 through 21-61 as shown in Figs. 10C, 10D and 10E. As a result, as shown in Figs. 11A, 11B, 11C and 11D which correspond to Figs. 4A, 4B, 4C and 4D, respectively, a 1152 ℓ 900 dot image is balanced at a center portion of the LCD panel 1. In Fig. 12, which is a modification of the scan line driving circuit 2' of Fig. 8, the start pulse signal ST is usually supplied to one of the flip-flops 21-0 through 21-511 on an upper-half side of the LCD panel 1, not to the shift registers 21-512 through 21-1023 on a lower half side of the LCD panel 1. Therefore, in Fig. 12, the switching circuits 22-512 through 22-1023 of Fig. 8 are not provided. In this case, the output of a decoder 23' is comprised of 512 bits, and therefore, the address signal ADD is comprised of 9 bits. Therefore, in this case, as illustrated in Fig. 13, a look-up table 65' whose content is shown in Fig. 14 is provided instead of the look-up table 65 of Fig. 8. In the above-mentioned embodiment, although the address signal ADD is generated from the look-up table 65 or 65', the address signal ADD can be generated by a microprocessor which can calculate the equation (2). As explained hereinbefore, according to the present invention, even an image having a smaller size than an LCD panel can be displayed at a center portion of the LCD panel.

Claims: An apparatus for driving a liquid crystal display panel (1) having M signal lines (SG j ), N scan lines (SL i ) (M, N=2, 3, ···) and M × N liquid crystal cells (C ij ) each connected to one of said signal lines and one of said scan lines, comprising: a plurality of serially-connected D flip-flops forming a shift register (21-0, 21-1, ...), connected to said scan lines, each for driving one of said scan lines; means (5) for generating a start pulse signal (ST) for defining a start scan line among said scan lines in synchronization with a horizontal synchronization signal (HSYNC); characterized by further comprising: a switching circuit (22-0, 22-1, ···) between each two D flip-flops, at least in the upper half of said shift register, for writing the start pulse signal in one of said D flip-flops, said switching circuits being connected to said start pulse signal generating means; means (6, 23, 23'), connected to said switching circuits, for selecting one of said switching circuits; and means (5), connected to said shift register, for generating a scan clock signal (CK) and transmitting it to said shift registers, to shift the start pulse signal through said shift register. An apparatus as set forth in claim 1, wherein each of said switching circuits comprises: a first AND circuit (221) having a first input connected to a prestage one of said D flip-flops; a second AND circuit (222) having a first input connected to said start pulse signal generating means; and an OR circuit (224) having inputs connected to the outputs of said first and second AND circuits, one of said first and second AND circuits being enabled by said selecting means and the other being disabled by said selecting means. An apparatus as set forth in claim 1, wherein said selecting means comprises: means (61, 63) for calculating a horizontal frequency f H in accordance with the horizontal synchronization signal; means (62, 64) for calculating a vertical frequency f V in accordance with a vertical synchronization signal (VSYNC); and means (65), connected to said horizontal frequency calculating means and said vertical frequency calculating means, for calculating an address (ADD) in accordance with the horizontal frequency f H and the vertical frequency f V , to thereby select one of said switching circuits in accordance with the address. An apparatus as set forth in claim 3, wherein said horizontal frequency calculating means comprises: a frequency-to-voltage converter (61) for receiving the horizontal synchronization signal; and an analog-to-digital converter (63), connected to said frequency-to-voltage converter. An apparatus as set forth in claim 3, wherein said vertical frequency calculating means comprises: a frequency-to-voltage converter (62) for receiving the horizontal synchronization signal; and an analog-to-digital converter (64), connected to said frequency-to-voltage converter. An apparatus as set forth in claim 3, wherein said address calculating means comprises a look-up table. An apparatus as set forth in claim 3, wherein said address calculating means calculates the address ADD by ADD=(N-f H /f V )/2. An apparatus as set forth in claim 3, wherein said selecting means further comprises a decoder (23, 23'), connected to said address calculating means, for generating a selection signal and transmitting it to one of said switching circuits.

Inventors: SHIKI TATSUYA, SHIKI, TATSUYA, SHIKI, TATSUYA, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: G09G3
IPC codes: G09G3, G02F1